Cosmic calendar fact

Life Signals

The earliest life signals were tiny forms of life, not animals or plants. Possible early life appears in September.

Wonder question What counts as evidence when ancient life was tiny and hard to see?

On the cosmic calendar

When the universe's story is shown as one year, this event can be compared with everything else on the same scale.

Science time
about 4 billion years ago
Calendar time
September 17th, about 04:52
Date confidence
approximate date

What happened?

Scientists have evidence that life may have been on Earth as far back as about 4 billion years ago. Early life would have been simple and microscopic.

How do we know?

Scientists study ancient rocks, chemical traces, and tiny clues left in very old Earth materials. NASA Astrobiology explains why early-life evidence is careful and difficult work.

What is still changing?

Very early life evidence can be hard to interpret. That is why this page says life signals and possible early life, rather than pretending we know one exact first organism.

Why it matters

Ancient life did not leave easy clues like big bones. The early evidence is tiny and indirect, so careful words matter: possible life signals tell us what scientists can support without pretending to know more than the evidence allows.

Image credit: NASA/JPL-Caltech. Background source: Age-Defying Star.